Use current checkout action in workflows

Author: ripred

.github/workflows/arduino-lint.yml

@@ -10,7 +10,7 @@ jobs:
     runs-on: ubuntu-latest
     timeout-minutes: 5
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - uses: arduino/arduino-lint-action@v1
         with:
           library-manager: update

.github/workflows/arduino_test_runner.yml

@@ -12,7 +12,7 @@ jobs:
     timeout-minutes: 20
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - uses: ruby/setup-ruby@v1
         with:
           ruby-version: 2.6